It is a party, usually done in someone's flat or house,(or at someone's villa, in the case of rich teenagers whose parents are on holidays),very popular in the maltese islands. During this party, people consume a lot of alcohol,binge drinking, in international terms, to the point of oblivion,and of becoming hara and end up sleeping in someone's bath and get found out asleep there when the organiser's flat mate wakes up for a pee.
The reason why malta is overpopulated,and why there is a lot of single mothers.

I heard that Mark and Maria where at a hara party at Erika's place and the landlord was not too happy about it.
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Rich Spoilt guy: Ih my parents will be on Holiday, Ejja Namlu hara party and we become all hara.
(Let us make a hara party and get all drunk, in typical maltese snob slang)
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Son: Mommy. how was I born
Mommy: You were conceived during a hara party.